Field of the Invention
It is conventional practice in the operation of an actuation and control circuit of an automatic transmission to vent hydraulic fluid from the circuit to a low pressure drain or sump at various locations throughout the circuit. Various friction elements of the transmission, i.e., hydraulically actuated clutches, brakes and servos, are engaged and released to produce gear ratio changes. Hydraulic fluid at elevated pressure is supplied to several friction element, each having a hydraulic piston located within a cylinder, thereby causing frictional contact among stacked friction plates connected to the driving and driven members.
When the friction element is disengaged during a gear ratio change, the torque capacity of the friction element is lowered by venting hydraulic fluid from the cylinder to a low pressure sump located below the circuit. While venting the portion of the hydraulic circuit that connects the friction element to the source of line pressure, it is possible for air to enter the passages of that portion of the circuit. Thereafter, when a gear ratio change requires reengagement of that friction element, air within those passages is either displaced by hydraulic fluid supplied to the friction element or is compressed within the passages by the pressurized hydraulic fluid traveling to the oncoming friction element. This process of displacing air or compressing air within the passages produces a detectable delay between placement of the gear selector mechanism by the vehicle operator in the desired position and completed engagement of the friction element that produces the selected gear ratio. This delay is frequently unacceptable to the vehicle operator.
Movement of the gear selector lever by the vehicle operator is linked to a manual valve, a component in the actuation and hydraulic control circuit that is supplied with regulated line pressure and directs line pressure selectively to output ports in the manual valve that correspond to the position of the gear selector lever. For example, if the gear selector is moved to a drive range (D or OD), the manual valve connects the regulated line pressure source to an output port through which all of the passages of the hydraulic circuit required to be pressurized to produce the selected gear ratios are pressurized. When the gear selector is moved from a forward drive position to the reverse position, the manual valve closes the output port that supplies pressure to the passages requiring pressurization in the forward drive condition and instead connects the line pressure source to an output port that pressurizes the lines required to be pressurized to produce reverse drive. However, if the pressurized fluid in the offgoing friction element is permitted to drain to a low pressure sump, delay in engaging the desired gear ratio after moving the selector lever to the selected position will occur nonetheless.
Various attempts have been devised to avoid this delay. For example, the manual valve of some automatic transmissions is constructed so that the drain passage is connected to the low pressure sump through a drain passage having an opening to the sump that is elevated in relation to the elevation of the manual valve and extends preferably to the highest point in the hydraulic circuit. Manual valves used in combination with the elevated exhaust passage require a long drain passage at one end of the manual valve and a corresponding length at the opposite end. These lengths contribute to increasing the overall size of the manual valve in comparison to a conventional manual valve, which merely connects the drain for exhaust passages directly to the low pressure sump. Consequently, additional space must be dedicated to the manual valve in the valve body, the compact compartment containing all of the control valves of the hydraulic control circuit. Space within the labyrinth produced by passages of the hydraulic circuit and valves connected by passages is at a premium; therefore, additional space for any valve such as the manual valve is undesired.